Selection of the T cell repertoire is initiated by the expression of rearranged TCR alpha and beta chains on immature thymocytes. The interaction of TCR and CD4/CD8 accessory molecules with the MHC molecules expressed in the thymus plays a key roll in the development of functional T cell immunity. The studies outlined in this proposal are to explore the interaction of CD4/CD8 molecules, TCR and MHC molecules both in vitro and in vivo to obtain a better understanding of T cell repertoire selection. In preliminary studies we have demonstrated that: a) the Mls and I-E reactivity of certain TCR V/Beta chains, as defined by deletion of T cells expressing these V/Beta in the thymus, does not correspond to the antigen specificity of mature clones expressing the same TCR in vitro; b) CD4-CD8+T cells bearing I-E reactive V/Beta chains often escape negative selection in vivo in I-E mouse strains; and c) the V/Beta 8.1+ TCR that can react to Mls on CD4+CD8- and CD4-CD8- hybridomas fails to react to Mls when CD8 is expressed in the absence of CD4. These findings indicate the need to further define the role of CD4 and CD8 in the development of the T cell repertoire in the thymus and in the antigen specificity of mature T cells. The specific aims are: 1) to determine how the differential expression of CD4 and CD8 molecules alters the functional specificity of Mls-reactive T cells; 2) to determine why some TCR Beta chains whose specificity can be demonstrated by virtue of their deletion in the thymus do not display and negative selection events occur at the CD4+CD8+ phenotype; and 4) to determine whether all positive and negative selection events occur at the CD4+CD8+ stage of thymocyte development and whether these events can operate through CD4 and CD8 independently. In proposed experiments are designed to provide a better understanding of the cellular and molecular interactions operating on both negative and positive selection of T cells in the thymus.